1. Field of the Invention
The present invention concerns a method for magnetic resonance imaging of an examination subject on the basis of a partially parallel acquisition (PPA) with multiple component coils, as well as a corresponding magnetic resonance system.
2. Description of the Prior Art
By the use of parallel imaging techniques with multiple component coils operating in parallel (Parallel Acquisition Techniques, PAT) in magnetic resonance imaging the detection time—known as the acquisition time (TA)—can be markedly reduced, or given the same acquisition time a markedly higher resolution can be achieved. This occurs by a reduced number of k-space lines being acquired (for example in the phase coding direction in spatial frequency space) than is typically necessary in order to satisfy the Nyquist theorem. Such techniques are known under the designation “partially parallel acquisition (PPA)”. In order to reconstruct images with the same resolution from the undersampled k-space data, undersampled (and thus incomplete) data sets are acquired with the use of an arrangement of multiple coils (known as local or component coils), and reference lines. The reference lines are respectively acquired by each of the multiple component coils and serve as calibration data points in order to adapt the incomplete data sets of different component coils to one another in order to generate complete image data. For example, the reference lines can be acquired with lower frequency in a middle region of k-space. In the prior art, appropriate reconstruction methods are known that normally represent algebraic methods, for example SENSE (Sensitive Encoding) and GRAPPA (Generalized Autocalibration PPA).
For static examination subjects or static tissue segments, the partially parallel acquisition and reconstruction techniques are very well developed, and qualitatively high-grade, artifact-free images can be created with conventional partially parallel acquisition.
Partially parallel acquisition is particularly suitable for the acquisition of moving tissue because the shortened acquisition time can reduce movement artifacts. While the acquisition time for 2-dimensional images is often fast enough to freeze the movement, for volume or 3D acquisitions the acquisition time is generally not short enough in order to acquire the entirety of the data within a sufficiently small fraction of a typical periodic movement (for example a breathing cycle) so as to acquire images without movement artifacts. In order to shorten the acquisition time for a single high-resolution 2-dimensional or 3-dimensional image, the reference lines can be acquired separately beforehand in a partially parallel acquisition. This acquisition technique can reduce the acquisition time of an image in k-space, but it must be insured that the information in the reference lines must be coherent with regard to the subsequently acquired image information in order to be able to implement a reconstruction of the partially parallel acquisition. If portions of the examination subject are at different locations during the acquisition of the reference lines and the acquisition of the image itself, ghost artifacts can arise. Artifacts can likewise arise if the state of the measurement system changes between the acquisition of the reference lines and the acquisition of the image data (i.e. B0 field, shim, eddy currents, B1 field).